In the last 20 years, one type of fat in ­particular has been portrayed as being especially bad for us: ­saturated. We are submitted to a barrage of advice: "saturated fats are killing us"; "eat low-fat spread"; "don't use lard"; "trim off all the fat from meat". In the latest ­instalment of this long-running saga, heart surgeon Shyam Kolvekar is now demanding that we banish butter from our diets.

But no entire food group needs to be (or should be) ­completely removed from our diet and fat is no ­exception. Each cell in the body requires fatty ­acids to absorb ­vitamins A, D, E and K. Fat adds to the taste of food and is an ­important energy source. We need it to support our natural growth, to keep skin healthy and taut, to protect our vision and boost our immune ­functions. We need it to help our brains function ­normally, too. And we need to eat a healthy balance of naturally occurring fats to get these benefits.

So how much fat is safe? Fats should not take up more than 35% of one's daily calorie intake. That may sound like a lot, but fat is a very ­intensive ­calorie source. For example, 100g of butter – 10 of the small packs that ­cafes provide with a scone – provide 717 calories, making up one third of the average female's recommended daily calorie intake of 2,000 (men's is higher, at 2,500). And "good" fats don't contain any fewer calories than bad fats, so substituting an olive oil-based spread for butter won't make you slimmer (and it will likely be laden with colourants and other undesirable additives).

But there is more to fat than calories, so knowing your trans fats from your monounsaturates is one of the best things you can do for your health.

Saturated fat

This is the most traditional source of fat and is largely animal-based. It includes the fat found in meats, ­butter, cheese and cream. There are also many food products that contain these fats, with cakes, biscuits and ­pastries being the obvious culprits. Slightly less obvious are items such as barbecued chicken, many sauce mixes, pre-frozen roast vegetables and ­confectionary bars. Guidelines tell us that we should not exceed 11% of our daily fat intake in this form. It is seen as a bad fat because too much of it will contribute to hardening and narrowing of the arteries.

Unsaturated fats ('good' fats)

There are two main kinds of unsaturated fats – monounsaturated and poly­unsaturated. The former come mainly from the oleic acid found in most nuts, avocado pears, rapeseed and ­olive oils. Monounsaturated fat is believed to lower cholesterol and may assist in ­reducing heart disease. Like polyunsaturated fat, it provides essential fatty acids for healthy skin and the development of body cells. And polyunsaturates are the essential fatty acids that contain the richest sources of Omega-3 and Omega-6, mainly found in fish oil, sunflower and corn oils and products made from these oils. Polyunsaturates can help reduce the "bad" cholesterol caused by ­saturated fat. You will find good sources in cold-pressed, unfiltered organic oils such as flax and rapeseed as well as in fish oils.

Trans fatty acids (TFAs)

A by-product of hydrogenated vegetable oil, TFAs are technically monounsaturates, but have been proven to be dangerous to health. Denmark was the first country to cut them out completely, saying there was "no such thing as a safe limit". Switzerland and Austria have since followed. And this week the UK Faculty of Public Health, representing 3,300 doctors, is urging ministers to eradicate them here, too.

TFAs were originally created in 1903 when an American chemist worked out that by boiling cottonseed oil to 260C, he could make it solidify. Why did he want to do that? To make cheap candles. Proctor & Gamble spotted the potential for food and bought the patent in 1909. It soon launched Crisco shortening in the US, a wonder food that contained no animal fat, didn't soak up other ­flavours and had a great shelf life. It wasn't until the 1970s that researchers discovered how ­damaging TFAs were.

TFAs do not just lower your "good" cholesterol level; they simultaneously raise the "bad". The most important research began in the US in 1976 and ran for 13 years. It found that a 2% increase in TFA ­calories would bring a 23% increased risk of heart ­disease. So, in terms of heart health, consuming 25g of TFA is equivalent to eating an entire 227g block of butter. And something like a take-away pie could easily contain 15g of TFA.

These days, a voluntary code has drastically reduced the amount of trans fat in supermarket own-brand products, but look out for hydrogenated vegetable oil on the ingredients lists of familiar brand names. You will still find it in cakes and confectionary, ­instant hot drinks, ready-bake cake mixes and, most commonly, where it is unidentified: that is, in take-away foods (especially anything that has been deep-fried). Trace amounts of TFAs (less than 1%), naturally occur in meat and milk too, but these do not seem to carry any risk.

So our Victorian ­ancestors, it seems, had a point: stick to natural sources and just watch the quantities. There ­really is nothing wrong with ­butter as long as you spread it on in moderation.